1. Field of the Invention
The present invention pertains to the field of Inverter Circuits used to convert direct current, D.C., to alternating current, A.C. In general, such apparatus have been designed to receive a D.C. input which in turn is converted to an A.C. source suitable for driving an A.C. receptive load. The disclosed apparatus controls the inverter circuit by altering the magnetic characteristics of the transformer which supplies the base current to the inverter transistors or equivalently, the voltage to the source of MOSFET power transistors.
2. Prior Art
A common power inverter application is to provide compatible A.C. power to operate fluorescent lamps, Cold Cathode Fluorescent lamps, and electro-luminescent panels. Fluorescent lamps are commonly used to provide illumination, particularly in industrial environments where their economy of power utilization is highly desirable. Because of their greater efficiency in converting electricity to light, the cost of utilization is significantly reduced when compared to incandescent lighting.
Cold Cathode Fluorescent lamps (CCFLs) are used to backlight Liquid Crystal Displays (LCDs) in computer applications and Electro-luminescent (EL) panels are used to backlight LCDs, key switches, and other devices in many applications. Their popularity is due to high efficiency and small size. These devices require a high voltage ac current to drive them. This power is commonly supplied by power inverter circuits.
A common limitation of these devices has been that they have required sophisticated circuitry to vary the brightness of the above mentioned lamps. Most modern fluorescent lamp ballasts utilize a D.C. to A.C. inverter circuit to strike and supply operating power to the lamps. Many inverter circuits commonly supply a non-variable voltage to the load. As control circuitry is added to accomplish regulation or dimming of the light source, the complexity and cost has historically increased dramatically while the reliability and manufacturing consistency have decreased. Additionally the control circuitry often interacts in an undesirable manner with various aspects of the circuitry thereby requiring further complexity to compensate for these effects.
Likewise, switching power supplies and high frequency supplies for driving halogen lamps commonly suffer from the same limitations.
The present invention addresses the above limitations in several ways. The first is that the apparatus described herein is isolated and independent from the drive circuitry. It requires very few components and does not require complex feedback loops to control the inverter output level. Secondly when used to drive a fluorescent lamp load, a series resonant circuit comprised of an inductor and resonant capacitor is often used to boost the voltage level to that required to strike and operate the lamp. By adding a secondary to the series resonant inductor and rectifying the output into D.C., this voltage can be used to supply the control current on a delayed basis and thereby provide full start up power to the fluorescent lamp load independent of the setting of any dimming or level controls. Alternatively, in conjunction with a simple RC timer, the modified inverter circuit is also able to strike the lamps at a very low dimming level.